In the 1980's there has been an explosion of new plastic containers in the marketplace. Many of these containers are made from polyolefins such as polypropylene and polyethylene because of their relatively low cost, good heat sealability and relatively high moisture resistance. However, these polyolefins exhibit a fairly high permeability to gases, including oxygen, so that, used alone, they are not adequate for packaging oxygen-sensitive foods and other materials that degrade in the presence of oxygen.
To impart oxygen impermeability to polyolefin packages, various barrier materials, such as ethylene vinyl alcohol copolymers, vinylidene chloride and nylon are used in multi-layer laminates. Such laminates are well known and have been the subject of many patents. Containers can be formed directly from large sheets of these materials, or, alternatively, individual billets may be used instead of sheets.
It is recognized that to achieve significant market acceptability, such packages or containers must be able to withstand heating without significant distortion. The ability to "hotfill" packages or to retort foods after they have been packaged permits the foods to be stored without refrigeration. Commercial sterilization by hotfilling or retorting imposes several additional restrictions on the choice of materials for the package: (1) the heat seal must survive commercial sterilization temperatures of over 180.degree. F. or typical retort conditions of steam or water at 250.degree. F. or more under pressure for one half hour or more; (2) the structure must not delaminate, unduly shrink or wrinkle as a result of the sterilization; (3) the oxygen and water barrier properties must not be permanently adversely affected by the conditions of commercial sterilization; and (4) the structure must be adequately strong to permit the handling of the package while still hot. The need for these additional hotfilling or retorting requirements rules out many of the materials and structures commonly used for non-retorted film or sheet food packages.
One very successful method of forming polymers below the melting point of the polymer is offered by Shell Oil Company and is called solid phase pressure forming (SPPF). In the SPPF method, manufacturers work with polyolefins below their melting point. The process begins with polyolefin pellets which are extruded into sheet 0.25 to 2 mm thick, depending upon the application. The sheet is fed through a radiant heating unit and then to the thermoformer which produces finished containers and other parts. Since in the SPPF process the part is formed at a temperature below the polyolefin melting point, accurate heating control is required to produce sheet uniformly heated below the melting point for satisfactory forming. The heated sheet is clamped over a mold cavity. A forming plug then pushes the sheet towards the bottom of the cavity while air pressure forces the sheet over the contours of the mold. The newly formed shape cools almost instantly and becomes rigid enough for trimming in place or in a subsequent downstream trim operation. The remaining web is then ground for recycling. Representative of patents dealing with the SPPF process include U.S. Pat. Nos. 3,859,028; 3,499,188; 3,606,958; 3,546,746; 3,532,786; 3,642,415; 3,757,718 and 4,172,875.
Another method of forming polymers below the melting point of the polymer is described by the Budd Company in U.S. Pat. No. 4,014,970. Still another solid phase process is described and is referred to as a scrapless forming process. This process is Dow Chemical Company's process and is described in U.S. Pat. Nos. 3,739,052; 3,947,204; 3,995,736 and 4,005,967. In these processes, a plastic blank approximately equal in weight to the finished plastic container is cut from sheet stock material and processed by high pressure techniques to form the desired container. Ball Corporation employs a blank or billet solid phase process, as exemplified in U.S. Pat. Nos. 4,286,000 and 4,419,412.
Polyolefin, especially polypropylene, containers formed under typical solid phase forming conditions are highly oriented resulting in increased stiffness, impact strength and clarity. These are desirable qualities but orientation can be detrimental in some applications. When oriented polypropylene food containers are exposed to high temperatures, such as in a retort process, the orientation in the polymer relaxes and causes unacceptable container distortion. Problems with stress relaxation do not occur if the polymer is formed under melt phase conditions because the polymer does not become as extensively oriented. The objective of the hardskin/softcore approach of the present invention is to enable the sheet or billet to behave in the process machinery like a solid but to form as if it were in the melt phase.